By David Day
Mil-dot reticle scopes were originally designed for use by the United States Marine Corps snipers and were used extensively in the Viet Nam war as a means of determining distance to long-range targets. The Mil in mid-dot stands for milliradian. As the intent of this article is to discuss use of the Mil-Dot scope in airguns and not confuse the reader with scientific jargon, lets leave it at that. If you want to learn more about the physics of how and why the mil-dot evolved as it did, fire up your computer and go to http://www.boomershoot.org/general/TruthMilDots.htm
The theory and practice of mil-dot range finding are simple. If one knows the approximate height of a target and the distance between two dots (center to center) on a mil-dot reticle at a given range and magnification, it is possible to determine, with great precision, how far one’s target is positioned downrange. From this information, sight holdover can be rapidly determined with precise shot placement at the estimated distance. That sounds like a real advantage for the sniper who has to deal with a bullet drop of several feet over a thousand yards or more!
The standard mil-dot is designed so that the distance between any two dots (center to center) on the scope reticle at 1000 yards is 36 inches. If a standing human, approximately 72 inches tall (6 ft), is observed through a mil-dot scope at 1000 yards, and the crosshairs are placed at the very top of the person’s head, that person’s feet should just touch a position on the reticle that is at the center of 2 dots down on the vertical element of the reticle.
Knowing that the person is approximately 72” tall, and that one mil (the distance from dot center to dot center on the reticle) covers 36 inches at 1000 yards, one can deduce that a person subtending the a total distance between 3 dots is 1000 yards away (the counts the crosshairs as one dot center).
Image of a 6’ man at 1000 yards covers a distance of 2 dots – center to center.
Now, let’s suppose we are again looking at a standing human through a mil-dot equipped riflescope. After placing the crosshairs at the top of the person’s head, it is observed that this person’s feet are 4 dots down from the crosshair of the reticle. Assuming we are looking at a person who is approximately 6 feet tall, we can deduce that the distance covered between any two dots at the range at which the person is being observed is 18 inches (72”/4 dots).
Again, knowing that the distance between any two dots at 1000 yards is 36 inches and that our reference point has indicated that the distance covered between two dots is 18 inches, simple algebraic math tell us that the distance to the target is 500 yards away.
X yds/18” = 1000 yds/36”
X= (1000*18)/36
X= 500 yards
Mil-Dots and Airguns
The intended method of use for a mil-dot scope is all well and good if the target is several hundred yards away. But what about airguns, where the range is typically less than 100 yards?
The good news is, that as the range to a target decreases, the distance covered between two dots also proportionally decreases. At 100 yards, the distance covered between two dots on a mil-dot riflescope is only 3.6”, and at 50 yards, that distance is only 1.8 inches! If you happen to be lucky enough to be shooting an airgun that has a trajectory with a pellet drop of 1.8” from line of sight at 50 yards, simply align the first dot below the crosshairs on the target and you should be dead-on at 50 yards.
This utilization of the mil-dot technology for an airgun is a bit different that the way it was intended to be used in high-powered rifles at great distances. If you have an object of known size to shoot at, say a 2 inch diameter plastic ball or a 1.5” squirrel head, a mil-dot scope could be used as it was traditionally intended, but the math gets a bit cumbersome. And for every shot you make you do not want to go through the process as outlined above.
However, as long as you can fairly accurately estimate the distance to your target, either by experience or with the assistance of a range-finding device, the dots on your scope’s reticle can eliminate all significant estimation in hold over/under and totally free you of the need to change your scope’s elevation setting for the distance at which you are shooting.
All that this process requires is knowledge of your pellet’s trajectory and a little bit of understanding about the relationship between your target distance and how that translates into measurements on the mil-dot scale. Just keep in mind that at 100 yards, the distance between two dots (center to center) on the mil-dot reticle is 3.6 inches. Therefore the following relationships at various distances will be constant:
|
Range to target |
distance between dots |
|
100 yards |
3.6” |
|
75 yards |
2.7” |
|
50 yards |
1.8” |
|
25 yards |
0.9” |
|
12.5 yards |
0.45” |
|
10 yards |
0.36” |
So, if you are shooting at a target that is at 60 yards (distance between mil-dots = ~ 2.2”), and you know that your point of impact is roughly 4 inches below the point of aim at that distance, you would want to hold over on your point of aim by almost 2 dots so that the 2nd dot down from the crosshairs is just below the target.